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Review

Calcium-Based Metal–Organic Frameworks and Their

Potential Applications

Shikai Xian, Yuhan Lin, Hao Wang,* and Jing Li*

Dr. S. Xian, Y. Lin, Prof. H. Wang, Prof. J. Li

Hoffmann Institute of Advanced Materials

Shenzhen Polytechnic

7098 Liuxian Boulevard, Shenzhen, Guangdong 518055, P. R. China

E-mail: wanghao@szpt.edu; jingli@rutgers

Dr. S. Xian, Prof. J. Li

Department of Chemistry and Chemical Biology

Rutgers University

123 Bevier Road, Piscataway, NJ 08854, USA

The ORCID identification number(s) for the author(s) of this article

can be found under doi/10.1002/smll.202005165.

DOI: 10/smll.

1. Introduction

Metal–organic frameworks (MOFs) are crystalline solids featuring

extended networks formed by coordinate bonds between inor-

ganic nodes (metal ions or clusters) and organic linkers.[1] MOFs

are characterized by their structural diversity and high tunability

with respect to porosity and surface functionality. As a relatively

new class of multifunctional crystalline materials, MOFs exhibit

considerable promise for applications across a broad range of

technologies, including gas storage,[2–5] molecular separation,[6–14]

chemical sensing,[5,15,16] heterogeneous catalysis,[17–20] and energy

efficient lighting technologies,[21–23] to name a few. For certain

applications, MOFs have outperformed some of those traditional

inorganic or organic materials, and thus, hold great promise as

their replacements or supplements. For example, over the past

few years, several MOFs have been reported to be capable of

full separation of light hydrocarbons (e., ethane/ethylene,

Metal–organic frameworks (MOFs) built on calcium metal (Ca-MOFs) rep-

resent a unique subclass of MOFs featuring high stability, low toxicity, and

relatively low density. Ca-MOFs show considerable potential for molecular

separations, electronic, magnetic, and biomedical applications, although they

are not investigated as extensively as transition metal-based MOFs. Com-

pared to MOFs made of other groups of metals, Ca-MOFs may be particularly

advantageous for certain applications such as adsorption and storage of

light molecules because of their gravimetric benefit, and drug delivery due to

their high biocompatibility. This review intends to provide an overview on the

recent development of Ca-MOFs, including their synthesis, crystal structures,

important properties, and related applications. Various synthetic methods

and techniques, types of building blocks, structure and porosity features,

selected physical properties, and potential uses will be discussed and sum-

marized. Representative examples will be illustrated for each type of impor-

tant applications with a focus on their structure–property relations.

ethylene/acetylene, propane/propylene)

through highly selective size exclusion

mechanism,[6,11,24] which has not been

achieved by conventional adsorbent mate-

rials. The potential uses of MOFs depend

on their pore structure (pore size/pore

shape), surface functionality, as well as the

type of metal centers and ligands.

Calcium-based metal–organic frame-

works (Ca-MOFs) represent a subgroup of

MOFs with calcium as metal centers. Unlike

MOFs built on transition and post-transition

metals (Zr, Fe, Co, Ni, Cu, Zn, etc.) which

tend to form commonly observed SBUs and

topology and, therefore allow for successful

design and implementation of targeted

structure and functionality, the prediction

of coordination geometry and structural

topology of Ca-MOFs are much more chal-

lenging. This could be attributed to the fact

that the bonding interactions between cal-

cium and organic ligands (commonly carboxylates or phosphates)

are more ionic, and thus the coordination mode of calcium and

the topology of Ca-MOFs largely rely on the nature of the organic

ligands as well as the synthetic conditions. However, Ca-MOFs

possess several advantages compared to those built on transi-

tion metals: 1) Ca-MOFs generally feature high thermal stability

because of its high electropositivity which leads to strong, ionic-

like bonds with organic ligands (e., carboxylates). 2) Calcium

is earth-abundant (3% of Earth’s crust) and nontoxic, making

Ca-MOFs relatively inexpensive and environmentally safe, thus

especially promising for biological related applications. 3) The

lightweight of calcium metal offers gravimetric benefit for gas

adsorption/storage related applications.

To date, more than 150 Ca-MOFs have been reported (Table 1 ).

These MOFs bear different structural features and have been

evaluated for various applications, including molecular sepa-

ration, drug delivery and controlled release, chemical sensing,

and proton conductivity, to name a few. In this review, we

will present an overview of recent development of Ca-MOFs,

including their design, synthesis, crystal and pore structure,

important properties, and potential applications.

2. Synthesis and Structures of Ca-MOFs

2. Synthetic Methods

Similar to MOFs made of other metals, solvothermal synthesis

represents the most commonly employed preparation method

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Table 1. List of Ca-MOFs and selected properties.

Name Formula Dimension BET [m 2 g− 1 ] Properties tested Ref.

MUT-1 [Ca(NBDC) · DMF]n 3D 28 CO 2 adsorption [45]

Ca-SBF [Ca 2 (SBF-TCA)(DMF) 2 ] · 2DMF 3D 378 CO 2 adsorption [84]

Ca-SBF-1 [Ca 2(SBF-TCA) (CH 3 OH)2(H2O) 2 ] · CH 3 OH·4H2O 3D 444 Toluene detection [84]

Ca(squarate) Ca(SA) · 3H 2 O 3D 224 C 2 hydrocarbons separation [154]

UTSA-280 Ca(SA)(H2O) 3D 331 C 2H4/C2H6 separation [24]

CYCU-1 [Ca(SDB)] · 0 2O 3D 224 N 2, H2, CO 2 adsorption [35]

[Ca(SDB)] · H 2O 3D 145 CO 2 /N 2 separation [25,155]

SBMOF-1 Ca(SDB) 3D 145 C 1 /C2 hydrocarbons separation [39]

SBMOF-2 Ca(H 2 TCPB) 3D 195 C 1 /C2 hydrocarbons separation [39]

SBMOF-2 Ca(H 2 TCPB) 3D 195 Xe/Kr separation [38]

SBMOF-2 Ca(H 2 TCPB) 3D 220 Separation alkane isomers [8]

[Ca(1,4-BDC)(BPDO)] · 0 3D – CO2/CH4 selectivity [34]

CaBTB [Ca5(BTB)2(HBTB)2(H2O) 6 ] · (THF)12(H 2 O) 2 3D 914 H 2 and CO 2 adsorption [47]

Ca-5TIA-MOF Ca 2 (5TIA)2(H2O) 2 · DMF 3D – CO 2 adsorption, catalytic

hydrosilylation of benzaldehyde

[43]

CaFu – – – Fluoride adsorption [67]

MOF-1201 Ca 14 (l-LAC) 20 (acetate) 8 (C2H5 OH)(H2O) 3D 430 Carrier for a fumigant [42]

MOF-1203 Ca 6(l-LAC)3(acetate)9(H 2 O) 3D 160 – [42]

[Ca(BDCPO)(DMA) 2 ] · 2DMA 2D – Cu 2 + adsorption [59]
[Ca(HBTC)(H 2 O)] · 2H 2 O 3D – Photoluminescence, mercury removal [156]
Ca(HBTC) 2D 1 Hydrogen storage [32]
Ca 3 (BTC) 2 2D 1 Hydrogen storage [32]
Ca 3 (BTC) 2 (H 2 O) 12 2D – Photoluminescence [157]
Ca 2(BTC)(PZC)(H 2 O) 3 2D – Photoluminescence [157]

AEPF-1 [Ca(HFIPBB)(H 2HFIPBB)0(H 2O)] · 0 3 H 6O 3D – Hydrogenation of styrene [88]

AEPF-1 [Ca(HFIPBB)(H 2HFIPBB)0(H 2O)] · 0 3 H 6O 3D – Separation of organic solvents [158]

CaP1 Ca(H2O)3(HPXBP) 3D – Stimulates bone mineralization [104]

CaP2 Ca 2 (H 2 O)2(HPXBP)1 3D – – [104]

Ca-BDC – 3D 7 Controlled release of Curcumin [26]

Ca(BDC) – – Anodes for lithium-ion batteries [143]
Ca(BDC) · H 2 O, Ca(BDC) 3D – Anodes for lithium-ion batteries [159]
[Ca(BDC)(H 2 O) 3 ]n 1D – Photoluminescence [160]
[Ca(oBDC)(H 2 O)]n 2D – Photoluminescence [160]
Ca(H 2OLZ) · xH 2 O (x = 0, 2, 4) 1D, 2D, 3D – pH-triggered delayed release of olsalazine [123]

MIL-155 [Ca 2 (H2O)(H 2 GAL)2] · 2H2O 3D – Release gallate for in vitro antioxidant activity [121]

BioMIL-3 Ca 2(ABTC)(H2O)(DMF) · xH2O·yDMF 3D – Trap and deliver NO at a biological level [161]

Ca-Pam [Ca(H 2-PAM)(H 2 O)] · H 2 O 1D – Cytotoxicity against cancer cells [162]

Ca-Zol Ca(H2-ZOL)(H 2 O) 1D –

Ca-SMOF-1 [Ca2(5A-9YA-IPA)2(DMF) 2 (H 2 O)]n 2D – CL scintillation [81]

Ca-SMOF-2 [Ca 2(5P-1YA-IPA)2(DMF) 2 ]n · 0 2D –

CaS 6 C 6 – 2D – Electronic and transport properties [163]

Ca(ABDC)(DMF) 3D – Dielectric behavior [144]
Ca(l-TAR) · 4H 2 O 3D – Proton conductivity [134]
[Ca(OBPA)(H 2 O)2]n 3D – Photoluminescence [164]
[Ca(BD)DMF] · DMF · H 2O 3D – Photoluminescence [165]
[Ca 2 (ABTC)(H 2 O)2(DMA)] · 3H2O 3D – Photoluminescence [165]
[Ca(TTD)(Diox)] · 2H 2 O 3D – Photoluminescence [165]
[Ca 2(DBBD)(H2O) 2 (DMF)] · 2DMF · H2O 3D – Photoluminescence [165]

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Name Formula Dimension BET [m 2 g−1] Properties tested Ref.

[Ca 2 (TZDA)2(H2O)4] · 3H2O 2D – Photoluminescence [192]
Ca 2 (MDIP)(H 2 O) 4 ] · CH 3 OH · 4H2O 2D – Photoluminescence [193]
[Ca 3 (TCMB) 2 (H2O)8] · 3H2O 2D – Photoluminescence [194]
Ca(oBDC)(H 2 O) 2D 14 – [195]
Ca(mBDC)(H 2O)3 1D 4 – [195]
Ca(BDC)(H 2 O) 3 1D – – [195]
Ca(BDC-F4) · 4H2O 3D 2 – [27]
[Ca 4(BDC-F4)4(H 2 O) 4 ] · 4H 2 O 3D 348 CO 2/N 2 separation [196]
[Ca(BDC-F4)(MeOH) 2 ]n 3D – – [196]
[Ca(BDC-F4)(H2O)4]n 2D – – [196]
Ca(DHBQ)(H 2 O) 2 3D – Structure flexibility [197]
[Ca(μ3-PMDC)(H2O) 3 ] · H2 O 2D – – [198]
[Ca(HEDP)(H 2 O)] · 2 1D – – [28]
Ca(AEP) 2D – – [199]
Ca(OH)(AEP) · 2H 2O 2D – – [199]

GWMOF-7 [Ca(ADI)(H2O)2] · (BPY) 3D – – [200]

GWMOF-8 [Ca(ADI)(H 2O) 2 ] · (1,2-BPA) 3D – – [200]

[Ca(XYDPP)(OH 2)3]n 3D – – [201]
[Ca(mXYDPP)(OH2) 2 ]n 1D – – [201]
[Ca(oXYDPP)(OH2)]n 2D – – [201]

CAUMOF-4 Ca[2,6-PDC](H 2 O)1 1D – – [202]

CAUMOF-5 Ca[2,6-PDC](H 2 O) 2 1D – – [202]

[Ca 4(2,5-PDC) 4 (DMF)]n 3D – – [203]
[Ca(2,5-PDC)(H2O)]n 3D – – [203]
[Ca(2,5-PDC)(DMF)]n 3D – – [203]
[Ca(2,4-PDC)(H 2 O)]n 3D – – [203]
[Ca(2,4-PDC)(DMF)]n 3D – – [203]
[Ca(2,6-PDC)]n 3D – – [203]
[Ca 4 (3,4-PDC)4(H2O)]n 2D – – [203]
[Ca(3,5-PDC)(DMF)]n 3D – – [203]
[Ca(3,5-PDC)(H 2 O)2]n 2D – – [203]

Ca-PiPhtA-I Ca 2 [(PiPhtA)2]2−[(PiPhtA)(H2O) 2 ] · 5H2O 3D – Conductivity [42]

Ca-PiPhtA-II Ca 2 [(PiPhtA)2]2−[(PiPhtA)(H 2 O)2] 3D – Conductivity [42]

Ca 3 (HPA) 2 · 14H2O 0D – Corrosion inhibitor [204]
Ca(H-HPA) · 3H 2O 2D – – [205]
Ca 5(HPA)2-(H-HPA)2·6H 2 O 3D 3 – [205]

BioMIL-2 Ca(GLU) 3D – Structure transformation [206]

BioMIL-2-hyd Ca(GLU)(H 2O) 1D – Structure transformation [206]

CaH 6 DTMP · 2H2O 2D – Adsorption of H 2O/NH 3 [207]
Ca(2,5-Me-XYDPP) · 2H 2O 3D – – [208]
Ca(NO 3 )2(BDPPMBP) 2 2D – – [209]
[Ca(CIN) 2(PAL)]n 3D – – [210]
{(Ca(H2PZTC)(H 2 O)3)n 1D – – [211]

CPO-69-Ca Ca(DMBPDC) 3D – [212]

MFF-3 [C2-H6][Ca(H2O)2(DMF)] 3 · 1 3D – – [213]

Ca(H 2O)(ANI) 2 2D – – [214]
[Ca(TDZDC)(H 2O)2]n 2D – – [215]

Table 1. Continued.

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Table 1. Continued.

Name Formula Dimension BET [m 2 g− 1 ] Properties tested Ref.

[Ca(DPPP)2(DMF) 2 ] 1D – – [216]
Ca(BDHPPMB) · 2(CH 3 OH) 3D – – [29]
[Ca(CDC)(H 2O) 2 ] · H2O 3D – – [217]
[Ca(3-CBPP)2(H2O) 3 ]n 1D – – [218]
[Ca(3-CBPP)2(H 2 O)2]n 1D – – [218]
[Ca(3-CBPP) 2 (H 2 O)4] · BPY 0D – – [218]

BioMIL-4 Ca(ALE) · H2O 1D – – [219]

[[Ca(H2O) 2 (NBA) 2 ] · 2DMP]n 1D – – [220]
[Ca(H-TCMBT)(H 2O)2]n 2D – – [221]
[Ca6(TCMBT)4(H2O) 14 ] · (H2O) 3 3D – Structure transformation [221]
[Ca(PDyDP)2(H2O)]n 3D – – [222]
Ca 6(1,3-ADC)4(CO3)(OH)2(H2O) 14 3D 180 – [33]
2 ∞[Ca(IM)2(IM-H)2] 2D – – [223]
Ca(PBA) 2 1D – – [224]
Ca(ATZA) 2 (H 2 O) 4 0D – – [225]
[Ca(SUC)]n 2D – – [226]
[Ca(H 2O)2(MPA)2] · H2O 1D – – [227]
[Ca(H 2O) 2 (2-CPA)2] · 2H 2 O 1D – – [227]
[Ca(PYR) 2(NBA)2]n 1D – – [228]
[Ca(H 2O) 2 (3-NPTH)] · H 2 O 1D – – [228]
Ca(l-TAR) 3D – – [229]
Ca(meso-TAR) 3D – – [229]
Ca(d,l-TAR)(H 2O) 4 3D – – [229]
Ca(H 2O) 2 (2-CA-4NBA) 2 1D – – [230]
Ca(H 2 O)2(PIDC) 2 2D – – [231]
[Ca(H 2O)(2-MeIM)(NBA)2]n 1D – – [232]
Ca(H2O)(NBA) 2 1D – – [233]
[Ca(N-MeIm)(NBA) 2]n 1D – – [233]
Ca(IM)(NBA) 2 1D – – [233]
[Ca 1(DEF)(μ-BDC)1]∞ 3D – – [234]
[Ca(μ-DMF)(μ-NDC)]∞ 3D – – [234]
[Ca(μ-DEF)(μ-TPDC)]∞ 3D – – [234]
[Ca(μ-DMF)(μ-DADC)]∞ 3D – – [234]
Ca(cis-4-CHDC) 2D – – [235]
Ca(2-DEPP-ES)(H 2 O) 2 2D – – [236]
(Ca2(OH)2(NDC) 3D – – [237]
Ca(H 2 O)(BPDC) 3D – – [237]
Ca(H2O) 3 (BPDC) 1D – – [237]

MPF-2 Ca[Z-l-Val-l-Val-l-Glu(OH)OH] 2D – – [238]

Ca(CA)(H2O) 3 2D – – [31]
Ca 2(CA)(C2O4)(H2O) 3D – – [31]
Ca(H 2 O)(1,2,4-BTC) 3D – – [239]
Ca(AHEDP) 4 1D – – [240]
[Ca(MALO) 2]n 3D – – [241]
Ca(BPP) 2 2D – – [242]
[Ca 2 (ODPP) 2 (μ-ODPP) 2 ] · (PhMe) – – – [243]
Ca(HBTC) · 2H2O 2D – – [244]

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challenging to design Ca-MOFs with targeted structures by fol-

lowing simple principles of reticular chemistry.

2. Types of Organic Ligands

Organic molecules with various functionalities have been used

for the construction of Ca-MOFs, which are summarized in

Scheme 1. These organic ligands can be grouped into three

main categories: 1) Carboxylates. Carboxylates, particularly

multicarboxylate ligands are the most common organic ligands

incorporated into MOFs. This type of ligands includes pure

carboxylates, and multifunctional molecules containing carbox-

ylates and other functional groups such as hydroxyl, imidazole/

triazole/tetrazole moieties, etc. It is noteworthy that while

transition metal-based MOFs, especially early transition metal-

based MOFs are mostly built on rigid, aromatic carboxylate

ligands, a number of flexible, aliphatic carboxylates have been

incorporated into Ca-MOFs. 2) Phosphates. There are very few

transition metal-based MOFs are made of phosphate ligands,

but they are commonly found in Ca-MOFs. This could be partly

due to the high charge density and ionic nature of Ca 2 + that

lead to strong bonding interactions with phosphates. 3) Others.

This category includes phenols, thiols, sulfates, imidazoles, etc.

In addition to small organic molecules, supramolecular ligands

such as CB[6] have also been incorporated into Ca-MOFs.

2. Representative Ca-MOFs

There are more than 150 Ca-MOFs reported so far, among

which ≈90 feature 3D frameworks and the others are 1D chain

or 2D layered structures. In this section, we will discuss some

representative Ca-MOFs, particularly 3D structures.

2.4. Ca(SDB)

The structure of Ca(SDB) (SDB = 4,4′-sulfonyldibenzoate)

was first reported by Parise and co-workers and Yang and co-

workers independently in 2012.[25,35] Both research teams

reported the synthesis, structure, and carbon dioxide adsorp-

tion of this Ca-MOF. The compound can be synthesized

through solvothermal conditions in ethanol or by microwave

irradiation in ethanol and water. The 3D structure is built on

calcium polyhedral chains composed of octahedrally coordi-

nated calcium centers, possessing 1D channels with an average

cross-section size of 5 × 5 Å (Figure 2 a). Ca(SDB) has a

Brunauer–Emmett–Teller (BET) surface area of 145 m 2 g− 1 , and

it selectively adsorbs CO 2 over nitrogen with an ideal adsorbed

solution theory (IAST) selectivity around 45. In a follow-up

study, Parise and co-workers [36] reported the crystal structure

of CO2-loaded Ca(SDB) and uncovered its selective adsorption

mechanism. It was revealed that the adsorbed CO 2 located in a

“pocket” formed by the linker molecules between two centroids

of the aromatic ring. In a more recent study, Banerjee et al.[37]

reported the separation of Xe and Kr by Ca(SDB). Through a

high-throughput computational screening, the authors identi-

fied Ca(SDB) out of 125 000 existing/predicted MOF structures

as the most promising material for the separation of Xe and

Kr. The subsequent experimental evaluation confirmed that

Ca(SDB) had the highest Xe/Kr selectivity among all porous

materials investigated. Ca(SDB) shows interesting proper-

ties for CO 2 capture and the separation of noble gases due to

Figure 1. a–g) Representative PBUs and SBUs in Ca-MOFs. Color scheme: green: calcium, gray: carbon, red: oxygen, blue: nitrogen, pink: phosphorus.

Hydrogen atoms are omitted for clarity.

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Scheme 1. Organic linkers used for constructing Ca-MOFs.

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Scheme 1. Continued.

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its high stability, but more importantly, its suitable pore size

and optimal surface functionality formed as a result of the

geometry of the organic linker.

2.4. Ca(H2TCPB)

Developed by Parise and co-workers,[38] Ca(H2TCPB) is a 3D

framework containing 1D channels with a diameter of ≈6 Å

(Figure 2b). In its crystal structure, each Ca 2 + is octahedrally

coordinated to six carboxylates from six different H 2tcpb

ligands which are partially deprotonated from H 4 TCPB. The

3D network is built on arrays of CaO 6 units interconnected

by H2TCPB. Ca(H 2 TCPB) is highly robust, which is thermally

stable up to 450 °C and retains its crystallinity well after three

years on shelf. Ca(H 2TCPB) selectively adsorbs Xe over Kr at

ambient temperature and pressure, with an IAST selectivity

of 10. The guest-loaded crystal structures were determined

by single-crystal X-ray diffraction analysis for Xe and Kr. As a

result of its larger atomic size and polarizability, Xe has a better

contact and higher occupancy compared to that of Kr, con-

sistent with gas adsorption results. In a follow-up study, [39] the

same research group studied the adsorption of methane and C 2

hydrocarbons on Ca(H 2TCPB) and uncovered structural flex-

ibility of the MOF. Compared to its guest-free structure, the

framework undergoes expansion upon inclusion of gases. More

recently, Li and co-workers[8] explored the use of Ca(H2TCPB)

for the separation of alkane isomers by making use of its frame-

work flexibility. As a result of its flexible structure, Ca(H 2TCPB)

exhibits a temperature/pressure-dependent adsorption behavior

toward different C 6 alkane isomers. It adsorbs linear hexane

but not branched isomers at 120 °C and 100 torr while at 60 °C

monobranched hexane can also be accommodated but not

dibranched isomers. Thus, a temperature-programmed column

separation process was designed and the mixture was success-

fully separated into individual isomers as a function of degrees

of branching.

2.4. Ca(C 4 O 4 )(H 2 O) (UTSA-280)

Ca(C 4 O 4 )(H 2 O) represents another 3D framework structure

built on Ca 2 + and squaric acid, featuring 1D ultramicropo-

rous channels (Figure 2c). The structure was first reported in

1987 by Weiss and Robl. [40] In its crystal structure, each Ca 2 +

is 7-coordinated to seven oxygen atoms from five different

C 4 O 42 − linkers and one terminal water molecule, adopting pen-

tagonal bipyramidal geometry. The Ca 2 + centers are bridged by

organic linkers to form 1D infinite chains which are further

connected to the resulting 3D framework. Recently, Chen and

co-workers [24] evaluated the separation of ethane and ethylene

by Ca(C 4 O 4 )(H 2 O), where it was named as UTSA-280. Interest-

ingly, the material adsorbs ethylene but fully excludes ethane,

resulting in complete separation of the two gases through

selective molecular sieving. Multicomponent column break-

through measurements confirmed that UTSA-280 is capable

of separating ethane and ethylene and producing ethane with

high purity. Very importantly, the material is highly stable and

easily scalable, making it promising for industrial separation

applications.

Figure 2. a–f) Crystal structures of representative Ca-MOFs. a,b) Reproduced with permission. [39] Copyright 2016, American Chemical Society. c) Repro-

duced with permission.[24] Copyright 2018, Springer Nature. d) Reproduced with permission.[34] Copyright 2015, Springer Nature. e) Reproduced with

permission.[41] Copyright 2017, American Chemical Society. f) Reproduced with permission.[42] Copyright 2014, American Chemical Society.

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OH, NN, NC(R), and NO 2 moieties are benefi-

cial for CO 2 capture.[44] Taking this into account, Akhbari and

Alavijeh[45] synthesized a new Ca-MOF, [Ca(NO2-BDC)·DMF]n

(MUT-1; NO2-BDC = 2-nitroterephthalate). It has a low surface

area of 28 m 2 g− 1 , but adsorbs 0 mmol g− 1 CO 2 at 298 K and

1 bar, higher than that of some reported MOFs, such as MOF-

and MOF-5.[46] Compared to MOF-2 and MOF-5, MUT-1 fea-

tures large amounts of NO 2 groups in its 1D channels, which

leads to higher affinity with CO2.

Monometallic Ca-based MOFs usually suffer from low

surface area and structural transformation upon activation/

adsorption due to the high coordination number and flex-

ible coordination geometry of Ca(II), severely limiting their

capacity for CO 2 capture. Using Ca and another metal such

as alkali metal or transition metal as metal center is an effec-

tive strategy to prepare heterometallic MOFs with higher

porosity.[47] Noh et al. prepared two porous Ca-based MOFs,

ZnCaBTB and CaBTB, composed of trinuclear Zn 2Ca and pen-

tanuclear Ca 5 clusters, respectively, and the metal centers are

linked by benzene-1,3,5-tribenzoate (BTB) units. BET surface

area of ZnCaBTB, the heterometallic MOF, is 1560 m 2 g− 1 ,

much higher than that of CaBTB (914 m 2 g− 1 ). For ZnCaBTB,

the adsorption capacity of CO 2 at 298 K and 1 bar is as high as

2 mmol g− 1 and the zero-coverage Qst value is 17 kJ mol− 1 ,

while CaBTB exhibited a lower CO 2 uptake (1 mmol g−1)

but a higher zero-coverage Qst (26 kJ mol− 1 ), which may be

ascribed to the lower surface area and the smaller pore size of

CaBTB. Another example of heterometallic Ca-based MOF is

[CdCa(PDC) 1(H2O)2]n (H 2 PDC = terephthalic acid), reported

in 2015 by Liu et al.[48] This MOF features rhombohedral open

channels of which the dimension is 9 × 9 Å and a large

BET area of 548 m 2 g− 1. It displayed a higher CO 2 capacity of

3 mmol g− 1 at ambient conditions which outperformed most

of Ca-based MOFs.

The adsorption capability of CO 2 of a MOF relates to its sur-

face area/pore volume, pore shape, pore size, as well as pore

surface functionality. It is well-recognized that the existence of

open metal sites (OMSs) on MOFs can boost the uptake of CO 2 ,

especially the adsorption capacity at ambient or lower pressure,

as a result of the strong interaction between the open metal

centers and the CO 2 molecules.[49–53] Saha et al. developed a

MOF based on alkaline earth metals and chelidamic acid (H 3 L)

with open metal sites, namely, [Ca 2 Na2(L) 2 (H2O)6]n·nH 2 O,[54]

which possesses a high BET surface area of 1419 m 2 g− 1 and

large pore volume of 0 cm 3 g− 1. Impressively, the CO 2 adsorp-

tion capacity of this MOF is as high as 3 mmol g− 1 at 298 K

and 1 bar, higher than that of all the reported Ca-based MOFs,

with a heat of adsorption of 35 kJ mol− 1. The authors attribute

the high CO 2 capacity to its large surface area coupled with the

strong quadruple interaction of CO 2 with the open metal sites.

Even at 0 bar, this MOF can still adsorb 1 mmol g− 1 CO 2 ,

making it a promising candidate for CO 2 capture from flue gas.

Developing MOFs with high CO2/N 2 selectivity is of great

importance for CO 2 capture from flue gas. For example, the

Ca(SDB) described in the above section demonstrates high

CO2/N2 selectivity. [25] It shows a reversible CO 2 uptake of

1 mmol g− 1 at 273 K and 1 bar, with a CO 2 /N 2 IAST selectivity

over 45. In spite of the absence of OMSs in the activated mate-

rial, Ca(SDB) exhibits a high interaction energy of 31 kJ mol− 1

for CO2 , comparable to that of MOFs with OMSs. Crystal struc-

ture of CO2-loaded Ca(SDB) revealed that the specific geom-

etry of the sulfonyldibenzoate creates a “pocket” where carbon

atoms from the CO 2 molecule are stably placed between two

centroids of the aromatic rings, and this configuration keeps

both oxygen atoms relatively close to two hydrogen atoms.[36]

Thus, the preferential adsorption of CO 2 by Ca(SDB) is a result

of its optimal pore size and pore shape.

Recently, Zaworotko et al.[55] reported an ultra-microporous

MOF Ca-trimesate, Ca(HBTC)·H 2 O (bnn-1-Ca-H 2 O, H 3 BTC =

trimesic acid) with a pore diameter 3 Å. It exhibits ultrahigh

CO 2 /N 2 (15/85), CO 2 /CH 4 (1/1), and C 2 H 2 /C 2 H 4 (1/1) selectivi-

ties of ≈1 000 000, ≈40 000, and ≈7000, respectively. Interest-

ingly, removal of the coordinated water molecules afforded

a narrow pore variant, Ca(HBTC) (bnn-1-Ca), which shows a

smaller pore diameter of 3 Å. The pore size of bnn-1-Ca is

larger than the kinetic diameter of H 2 but smaller than that of

many other gases, such as CO 2 , CH 4 , C 2 H 2 , and C 2 H 4. IAST

selectivity calculations for H 2 /CO 2 and H 2 /N 2 under equimolar

compositions afforded ultrahigh selectivities of 10 000 that

are consistent with its molecular sieving behavior. Due to the

high density of unsaturated calcium sites in the framework,

bnn-1-Ca exhibits a high H 2 capacity of ≈2 mmol g− 1 at 77 K

and 1 bar, making it a promising adsorbent for industrial H 2

separation.

3.1. Hydrocarbon Separation

Separation and purification of light hydrocarbon mixtures into

pure species represents a challenging industrial process but is

of paramount importance. Ca-MOFs investigated for differenti-

ating hydrocarbons are very limited, however, they have exhib-

ited great potential for this application.

An ultra-microporous MOF that is able to efficiently sepa-

rate C2H 4 and C 2 H 6 through molecular sieving was recently

reported by Chen and co-workers.[24] The MOF Ca(C 4 O4)(H 2 O),

namely, UTSA-280 has rigid 1D channels with cross-sectional

area of about 14 Å 2. The aperture could just enable ethylene

with cross-sectional area (13 Å 2 ) to diffuse into the frame-

work, and it acts as a molecular sieve to exclude the passage of

ethane with a larger cross-sectional area of 15 Å 2 (Figure 4 ).

Owing to the molecular sieving behavior, UTSA-280 achieved a

record-high C2H 4 /C2H 6 selectivity, which is remarkably higher

than those of Fe-MOF-74 (13)[10] and NOTT-300 (48). [56] Mul-

ticomponent breakthrough experiments further validated the

efficiency of this molecular sieve for the separation of ethylene/

ethane with high ethylene productivity under ambient condi-

tions. The coverage-dependent isosteric heat for ethylene was

in the range of 20–35 kJ mol− 1 , notably lower than that of

MOFs with OMSs, [10,57] implying that it is potentially practical

to regenerate this MOF under mild conditions.

Parise and co-workers [39] investigated the adsorption and

separation of C 1 and C 2 hydrocarbons using Ca(SDB) (denoted

as SBMOF-1) and Ca(H 2 TCPB) (denoted as SBMOF-2). Adsorp-

tion experiments of C 2 hydrocarbons on SBMOF-1 showed

a moderate adsorption of C 2Hn gases at 298 K, with uptakes

of 30, 30, and 29 cm 3 g− 1 for acetylene, ethylene, and

ethane, respectively. Methane is adsorbed at a lower amount

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of 18 cm 3 g− 1 at 298 K and 1 bar and IAST selectivity cal-

culation shows that the values for C2H6/CH 4 , C 2 H4 /CH 4 , and

C 2 H2/CH 4 are 74, 73, and 33, respectively. Heats of adsorption

of the three C 2 hydrocarbons are quite similar to each other

with the values of 34, 35, and 36 kJ mol− 1 for acetylene,

ethylene, and ethane, respectively, suggesting that there is no

significant difference with respect to the adsorbent–adsorbate

interaction. Single-crystal X-ray diffraction confirmed the main

hypothesized adsorbate–adsorbent interaction in SBMOF-1 is

CH⋅⋅⋅π. As expected, in the transient breakthrough simula-

tions, SBMOF-1 showed the capability of differentiating C 1 and

C 2 hydrocarbons, consistent with the gas adsorption results. As

for the aforementioned SBMOF-2, 17 cm 3 g− 1 of methane was

adsorbed at 298 K and adsorption capacities of 64, 59, and

62 cm 3 g− 1 were observed for acetylene, ethylene, and ethane,

respectively. The calculated C 2/C 1 selectivities are 26, 16, and 18

for C2H6/CH 4 , C 2 H4/CH 4 , and C 2 H2/CH 4 , respectively. These

values are lower than those calculated for SBMOF-1. The main

adsorbate–adsorbent interactions in SBMOF-2 are CH⋅⋅⋅π

and CH⋅⋅⋅O, as revealed by single-crystal X-ray diffraction

analysis. The calculated heats of adsorption are 30, 29, and

32 kJ mol− 1 for ethane, ethylene, and acetylene, respectively.

SBMOF-2 was further tested for the adsorption of heavier

C 3 –C 4 hydrocarbons. As expected, the corresponding results

coupled with that of C 1 and C 2 indicating that longer chains are

preferred over the smaller ones until the point of maximum

selectivity, when the entropic cost of the long chain ordering

affects the energy gained from the adsorption. [58] In situ XRD-

differential scanning calorimetry (DSC) results further sug-

gest that SBMOF-2 displays certain structural flexibility, which

allows the accommodation of all the C1–C 4 hydrocarbon gases

inside the pore space. Moreover, a seven-component (CH 4/

C 2 H2/C2H4/C2H6/C 3 H 6 /C 3 H8/C 4 H10) breakthrough experi-

ment was carried out and the results indicated that SBMOF-

has the ability to separate a seven-component mixture into four

different fractions, with increasing carbon numbers.

Making use of the flexibility of Ca(H2TCPB), Li and co-

workers[8] realized a complete separation of mono- and di-

branched C 6 alkane isomers through temperature- and

adsorbate-dependent size exclusion (Figure 5 ). Ca(H 2 TCPB)

adsorbed 88 mg g− 1 of nHEX but a negligible amount of 3MP

or 22DMB at 120 °C and 72 Torr, while, at 60 °C and 72 torr,

57 mg g− 1 of 3MP can be adsorbed on this compound but still,

the adsorbed amount of 22DMB was negligible. Similarly,

in the breakthrough experiment, ternary C 6 alkane mixture

can be separated into chemically pure individual components

by injecting the mixtures into the first column at 120 °C and

then through the second column at 60 °C. In view of results,

the author believed nHEX is capable of opening the pore suf-

ficiently large to allow it to enter at a significantly higher tem-

perature (120 °C) due to the smallest size and strongest binding

with the adsorbent. However, a lower temperature (60 °C) is

required for the larger-sized 3MP to enlarge the pore window

further for it to diffuse in. For the largest and least-binding

22DMB, this can be achieved only at a much lower tempera-

ture (30 °C). Finally, X-ray diffraction analysis was conducted

to provide further support of the correlation of the framework

flexibility and the selective adsorption of the MOF.

3.1. Noble Gases Separation

SBMOF-1 and SBMOF-2, elucidated in previous sections, are

two represetative Ca-MOFs studied for the separation of noble

gases. Through a high-throughput computational screening

and subsequent experimental evaluation, Banerjee et al. iden-

tified SBMOF-1 out of 125 000 existing/predicted MOF struc-

tures as the most promising material for the separation of Xe

and Kr (Figure 6 ). SBMOF-1 adsorbs 1 mmol g− 1 of Xe at

298 K and 1 bar, with a Henry constant of ≈38 mmol g− 1 bar−1, a

value that is substantially higher than those reported for other

MOFs, indicating its strong adsorption affinity toward Xe. In

contrast, its adsorption capacity and Henry constant are much

lower for Kr, leading to a high Xe/Kr selectivity of 16. Column

breakthrough experiments revealed that the material is capable

of separating Xe from other gases including O 2 , N 2 , CO 2, and

Kr, even with the presence of water. Single-crystal X-ray dif-

fraction analysis of Xe-loaded SBMOF-1 revealed that Xe is

adsorbed near the midpoint of the channel, interacting with

the channel wall composed of aromatic rings through van der

Waals interactions. The authors attributed the high Xe adsorp-

tion capacity and Xe/Kr selectivity of SBMOF-1 to its tailored

pore size that is optimal for Xe. SBMOF-2 [38] has pore walls

with phenyl rings with delocalized π-electron clouds and polar

Figure 4. a) Adsorption–desorption isotherms of ethane and ethylene in UTSA-280. b) Multicomponent column breakthrough curves for ethane and

ethylene. Reproduced with permission.[24] Copyright 2018, Springer Nature.

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transform infrared (FTIR) results revealed that the Ca 2 + ions in

this Ca-MOF are capable of being exchanged by Cu 2 + almost

quantitatively in seconds in aqueous solution. This MOF

showed a maximum Cu 2 + sorption capacity of ≈68 mg g− 1 at

pH = 7, which is higher than those of activated carbons,[60]

natural zeolite,[61] and several organic resins[62] (13–60 mg g−1).

The second-order rate constant k 2 of Ca-MOF is as high as

0 g mg− 1 min− 1 , obtained by exchange kinetics experiment.

The high capacity as well as the fast kinetics were attributed to

the fact that Ca 2 + forms primarily ionic bonds with carboxylate

ligands while Cu 2 + interacts via strong coordination bonds with

carboxylate groups. Thus, the stronger interactions of Cu 2 +

with the organic ligand is the driving force for the observed

rapid replacement of Ca 2 + by Cu 2 + ions.

A water-stable bimetallic MOF that is capable of selectively

capturing gold cations from electronic wastes was devel-

oped by Pardo and co-workers.[63] The MOF, Ca IICuII6[(S,S)-

me-thox] 3 (OH) 2 (H 2 O)·16H 2 O, possesses channels with thio-alkyl

chains from the natural amino acid l-methionine. It selectively

captures Au(I) and Au(III) salts (such as AuCl 3 and AuCl) in

the presence of a wide variety of other metal cations as the sulfur

atom from the thioether arm in the channel can efficiently

clutch Au(I) and Au(III) by forming coordinate bond. The

inductively coupled plasma atomic emission spectroscopy (ICP-

AES) and scanning electron microscopy (SEM) results reveal

that this MOF has an unprecedented high recyclable loading

of gold cations (15− 20 wt%) with fast kinetics. Additionally,

upon capture of Au(I) and Au(III), the MOF showed high cata-

lytic activity in cyclization/ketalization of 4-pentyn-1-ol. On the

basis of the well-known affinity of mercury to sulfur atoms,

the authors further extended the application of this MOF to

the capture and removal of Hg 2 + and CH 3Hg+ from aqueous

media.[64] As expected, this MOF is able to decrease the [Hg 2 +]

and [CH 3 Hg+] concentrations in potable water from highly

hazardous 10 ppm to the much safer values of 6 and 27 ppb,

respectively. Furthermore, when soaked in saturated solutions

of HgCl 2 (H 2 O) and CH3HgCl (H2O/MeOH 1:1) for 72 h, this

MOF can adsorb as high as 900 mg g− 1 of HgCl 2 and 166 mg g− 1

of CH 3 HgCl. In addition, it exhibited a highly preferential cap-

ture toward mercury over other cations including Na+, K+, Ca 2 +,

and Mg 2 +, and it was found that the capture process is fully

reversible as the mercury salts can be easily extracted with

dimethyl sulfide.

Ca-MOFs may be advantageous for food-related applications

because of their low toxicity compared to MOFs built on other

types of metals. A typical example is CaFu, a nontoxic calcium

fumarate MOF, which was tested for the removal of fluoride

from brick tea infusion. [65] Remarkably, its maximum adsorp-

tion capacity of fluoride in brick tea infusion is 166 mg g− 1

at 373 K, which is the highest value ever reported for fluoride

removal from brick tea infusion system.[66,67] Compared to

Tea-Al,[67] which suffers from low selectivity for fluoride, CaFu

preferentially adsorbs fluoride and nearly no catechins or caf-

feine is adsorbed when the dose is kept below 2 g L− 1. The

unsaturated metal sites where fluoride ions can be adsorbed

first is of key importance in the adsorption process, as was con-

firmed by FTIR and X-ray photoelectron spectroscopy (XPS).

Taking advantage of the naturally occurring lactate and ace-

tate linkers, Yaghi and co-workers fabricated an environmentally

friendly Ca-MOF Ca 14 (l-lactate) 20 (acetate) 8 (C 2 H 5OH) (H 2 O),

namely, MOF-1201.[41] Owing to the eco-friendly composi-

tion as well as the permanent porosity, MOF-1201 was then

studied as a fumigant carrier for 1,3-dichloropropene. Sorp-

tion of cis-1,3-dichloropropene by MOF-1201 was carried out

at 298 K and it revealed an uptake of 1 mmol g− 1 (13 wt%)

at a relatively low partial pressure (P/P 0 = 0). In the subse-

quent release experiments, liquid cis-1,3-dichloropropene was

released quickly, with 80% of the total weight evaporated within

1000 min g− 1 , while the cis-1,3-dichloropropene encapsulated

in MOF-1201 was released in a much slower manner, with

80% of the total (10 wt%) released in 100 000 min g− 1 , cor-

responding to 100 times slower release compared with liquid

cis-1,3-dichloropropene under the same conditions.

3. Photoluminescence (PL) and Sensing

PL is an important property of many MOFs, and their pho-

toemission can arise from different origins, for example,

ligand-based, metal-based, guest-induced, ligand-to-metal

charge transfer (LMCT), metal-to-ligand charge transfer

(MLCT), ligand-to-ligand charge transfer (LLCT), or combina-

tions of some of these.[5,68,16] Luminescent MOFs (LMOFs)

are promising as probing materials in the field of chemical

sensing and detection and as phosphors for lighting-related

applications.[5,21–23,69] Extensive studies have been performed

on LMOFs and related applications, including a number of cal-

cium-based LMOFs.

A 2D calcium-based LMOF with a dual-channel emitting

pathway was reported by Pan, Su, and co-workers. [70] The

MOF [Ca 3(HL) 2 (DMF) 5 ]n (H 4L = 2 ′-amino-[1,1′:4′,1′′-terphenyl]-

3,3′′,5,5′′-tetracarboxylic acid) (termed as LIFM-41) features

van der Waals layered structure. LIFM-41 displays excitation-

dependent PL and shows a rare overall emission color shifting

from blue to yellow and then to red region with continuously

change of excitation wavelengths (Figure 7 ). Dual-channel

emission pathways of LIFM-41 cause this uncommon phenom-

enon. The high energy emission below 500 nm was related to

the interlayer trapped excitons by the 2D layers after interligand

charge transfer, and the low energy emission above 500 nm

was ascribed to the intralayer formed excimers.[71] The layer

stacking by weak van der Waals forces enables exfoliation and

morphology transformation, which was achieved by ultrasound

in different ratios of DMF/H 2O solvents, or grinding under

appropriate humidity conditions, resulting in nanospheres,

nanobelts, or nanosheets. For nanosphere samples, the dual-

channel emissions were not affected greatly, only with reduc-

tion in intensities due to the enhanced dissipation process after

morphological transformation. The overall emission color of

the nanosheets is shifted to near white light compared to the

green emission of bulk crystals. This is because the layer num-

bers are notably cut down with respect to the thickness, and

therefore hampering the interlayer excitonic emission, leading

to disappearance of two channeled emissions in nanosheet.

Similarly, in nanobelt samples, the high energy interlayer exci-

tonic emission was almost totally diminished, and the overall

emission is redshifted to yellow region (maximum at ≈560 nm),

with dominant contribution from the excimers-related

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intralayer emission. This should be due to the fact that the

DMF-sustained multilayered structure no longer existed in

nanobelts with low thickness, and the interlayer emitting

channel is therefore tuned off. The successful manipulation of

dual-channel emission in LIFM-41 by reversible exfoliation and

morphology transformation prompts the conceptual application

as a writing-reading-erasing type optical memory, for which the

writing and erasing process can be realized by tuning on/off

the interlayer excitonic emission.

A Ca-MOF that undergoes a reversible photochromic trans-

formation from yellowish to dark green was reported by Zou

and co-workers.[72] The photochromic MOF [Ca 2 (BIPA-TC)

(DMF) 4 ]-2DMF (BIPA-TC = 5,5′-(1,3,6,8-tetraoxobenzo[lmn][3,8]

phenanthroline-2-7-diyl) bis-1,3-benzenedicarboxylate) features

a unique doubly interpenetrated 3D porous network. Upon

irradiation by sunlight for a few minutes, this MOF undergoes

a photochromic transformation from yellowish to dark green,

which returns to yellowish in a dark room for 2 d at ambient

temperature. The photoresponsive behaviors result from an elec-

tron transfer caused by π–π stacking interactions in the doubly

interpenetrated framework,[73] as manifested by powder XRD

(PXRD). Besides π–π electron transfer, the photochromic process

may also arise from the photoinduced radicals generated by the

ligand upon light irradiation,[74] confirmed by electron spin reso-

nance (ESR) spectra. Photoluminescence property of this MOF

was investigated as well. Under 505 nm excitation, this MOF

exhibited an emission centered at 575 nm. Since free H 4 BIPA-TC

ligand is not photoluminescent, the emission of the MOF should

be ascribed to LLCT and/or LMCT related to π–π stacking inter-

actions in the doubly interpenetrated framework.

Pan, Su and co-workers[75] reported a fluorescent MOF which

is able to maintain its blue emission at 12 MPa. The MOF

[Ca(TABD-COO)(DMF)] 2 (LIFM-40) is constructed from piezo-

fluorochromic ligand 4,4′-((Z,Z)-1,4-diphenylbuta-1,3-diene-1,4-

diyl)dibenzoic acid (TABD-COOH). LIFM-40 exhibited a ligand-

based emission.[76] The lifetime of LIFM-40 was measured as

2 ns, confirming that the emission is fluorescence. Notably,

the quantum yield (QY) of LIFM-40 is as high as 57%, which

is among the highest values for fluorescent Ca-MOFs, and a

value of 47% was observed upon compression under 8 MPa. As

the pressure increased, LIFM-40 preserved its blue fluorescence

all the time even after being compressed by 12 MPa and losing

crystallinity. This is because the rigidity of the framework pro-

tects the ordered packing mode formed by CH⋅⋅⋅π interaction

that helps constrain the rotation of phenyl rings and prohibit

π⋅⋅⋅π interactions against compression.

Figure 7. a) Excitation and b) excitation-dependent emission of LIFM-41. c) Schematic representation of the dual emission pathways (shown in peak-

deconvolution manner) of Ca-MOF bulk crystal and nanosphere samples: interlayer excitons-related emission (blue dash) and intralayer excimers-

related emission (red dash), and tuning off the former by cutting down layered structures in nanosheet and nanobelt samples by exfoliation and

morphology transformation. Reproduced with permission.[70] Copyright 2018, Springer Nature.

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exhibited relatively low selectivity with higher H 2 pressure.

Moreover, AEPF-1 also showed an effective catalytic behavior

in the heterogeneous hydrosilylation of a variety of molecules

including aldehydes, ketones, and alkenes with diphenylsilane.

The catalytic behavior of AEPF-1 could relate to the heptacoordi-

nated Ca environment, which aids coordination of the substrate

by increasing the coordination number to eight. Furthermore,

the acidity of AEPF-1introduced by the protonated ligand makes

the heterolytic cleavage of the hydrogen molecule easier. In a

follow-up study, the same group developed AEPF-3[91] featuring

fsh-type 3D structure. AEPF-3 demonstrated that complete

hydrogenation of styrene could be achieved in 5 h in toluene

at 373 K with 5 atm H 2, using 1 mol% catalyst. Given that the

mechanism of hydrosilylation is similar to that of the hydro-

genation reaction, AEPF-3 was also tested for the hydrosilyla-

tion reaction of benzaldehyde with diphenylsilane. The yield

of obtained silylated product was up to 85% after 22 h

(TOF = 1 h− 1 ) in toluene at 363 K with 10 mol% catalyst. Over

three cycles of reaction, the observed activity of AEPF-3 was kept

and its PXRD patterns were retained, indicating its high stability.

Ca-5TIA-MOF[43] was also studied as catalyst for hydrosilylation

of benzaldehyde. Ca-5TIA-MOF (10 mol%) can afford 70% yield

within 24 h, in catalyzing the hydrosilylation of benzaldehyde in

dimethyl sulfoxide (DMSO). The authors attributed its catalytic

activity to the following three points:[35,91] 1) a low restriction in

mass transport, 2) the Lewis acid character of the calcium, and

3) the presence of coordinated solvent molecules (e., DMF).

Only a minor detriment in the reaction (D < 10%) was observed

after the third run of the recycling experiments.

Benzaldehyde is an important intermediate for organic syn-

thesis of fine chemical products, widely used in medicine, dyes,

spices, resins, and other industries. However, benzaldehyde

is typically prepared by oxidation reaction of benzyl alcohol with

toxic metal oxides, peroxides, halides, and so on.[92–94] Thus,

the development of environmentally friendly catalysts is much

needed. Tai et al.[95] reported a Ca-MOF synthesized by one-pot

reaction with disodium 4-formylbenzene-1,3-disulfonate, isoni-

cotinic acid hydrazide, and Ca(ClO 4 ) 2 •2H2O and the compound

was then used as catalyst in oxidation of benzyl. A good con-

version of benzyl alcohol (78%) and excellent selectivity of ben-

zaldehyde (98%) was achieved when the reaction was carried

out at 130 °C in 1,4-dioxane. The coordinatively unsaturated cal-

cium could sufficiently contact with the reactant (benzyl alcohol

and O2), and the MOF can also promote rapid removal of the

product (benzaldehyde) as well as prevent further reaction of

the benzaldehyde. Using 2-carboxybenzaldehyde as organic

ligand, the author then synthesized another new structure,

[Ca(L)2(H2O)2]n (L = 2-carboxybenzaldehyde).[96] The MOF also

exhibited relatively good catalytic activity in the benzyl alcohol

oxidation reaction with moderate benzyl alcohol conversion

(53%) and benzaldehyde selectivity (83%).

Additionally, Tai and co-workers synthesized another

Ca-MOF [CaL 2(H 2 O) 2 ]n[97] by using 3,5-bis(4-pyridylmethoxy)

benzoic acid (HL) as ligand. The obtained MOF was used as

catalyst for the preparation of propargylamine in the A 3 cou-

pling reaction. The benzaldehyde conversion of 38% was

obtained over the MOF for the coupling reaction of benzal-

dehyde, phenylacethlene, and piperidine with 1,4-dioxane as

solvent at 120 °C for 12 h. The recovered catalyst worked well

up to four catalytic runs. In four successive cycles, the conver-

sion of benzaldehyde was 38%, 35%, 32%, and 29% at

120 °C for 12 h, respectively.

Two halide salt ligands, 1,3-bis(carboxymethyl)imidazo-

lium bromide (BCMIM·HBr) and BCMIM·HCl, in combi-

nation with calcium salt, formed the corresponding MOFs

termed as bcmim-Ca1 and bcmim-Ca2, respectively.[98] With

these two MOFs as catalysts, Pastor and co-workers tested the

Friedländer reaction of 2-aminobenzaldehydes. A full conver-

sion was achieved after 90 min in 10 equiv. pentane-2,4-dione

at 80 °C, using 10 mol% bcmim-Ca1 catalyst. The resultant

quinoline was isolated in pure form with a yield >99%, which

highlighted the potential use of this catalyst. This study also

demonstrated that small variations in the MOF structures, such

as the counterions of the organic linkers, may influence the

catalytic activity of the heterogeneous catalyst. Besides, the cata-

lytic systems work for not only 2-aminobenzophenones but also

2-aminobenzaldehydes, which had not previously been proceed

in the presence of MOFs for quinoline synthesis.

Polyoxometalates (POMs) have been demonstrated as one

type of new potential photocatalysts in the degradation of

organic dyes.[99,100] However, typical POM salts are generally

water-soluble and may induce secondary pollution if they are

directly used as photocatalysts. Li and co-workers [101] developed

a new MOF, [Ca(HL) 2(L)0 (H2O)4] [SiMo 12 O40]·5CH3CN·H 2 O

(L = 1,4-bis(pyridinil-4-carboxylato)-l,4-dimethylbenzene), through

slow diffusion reaction of the ligand into Ca 2[SiMo 12 O40 ]·nH 2 O.

The obtained MOF not only retained the photocatalytic prop-

erty of POM but also possessed an insoluble structure which

can avoid the secondary pollution in water. This MOF was used

for catalyzing the degradation of Rhodamine-B (RhB) under

UV irradiation as heterogeneous photocatalyst. With the pres-

ence of the MOF, photocatalytic activity increases from 35%

(without catalyst) to 91% after 90 min of UV irradiation.

3. Drug Delivery and Other Biomedical Applications

Ca-MOFs are particularly promising for biomedical-related

applications due to their low cost, nontoxic nature, and abun-

dant presence in the body (≈1 kg in the average human body)

together with its high recommended intake (≈ 1 g d− 1 ).[102,103]

Besides, many drugs or prodrugs are organic acids which could

be used as ligands. Thus, diverse bioactive MOFs combined

with Ca 2 + and drugs could be obtained, and it is possible to

realize intracorporal precise supplying of drugs through con-

trollable degradation of bioactive MOFs.

Ca(H 2O)3(HPXBP) (termed as CaP1, PXBP: p-xylylenebispho-

sphonate) represents a bio-Ca-MOF which was investigated for

its bioactivity. [104] The results suggested that CaP1 is able to pro-

mote the formation of bone-precursor phases. Further assess-

ment using the osteoblast-like MG63 cell line confirmed its

bioactivity and the results indicated that CaP1 could promote

in vitro bone-like mineralization, potentially useful for treating

bone disorders, such as osteoporosis. In a separate study, using

the same ligand PXBP, Rocha and co-workers [105] synthesized

a new bimetallic MOF [SrCa(H2O) 3 (H2PXBP)] (SrCaPAEM),

which was used in stimulating osteogenesis. The hydrolyzed

ligand in the MOF structure does not appear to influence the

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cells although the commercial molecule is toxic, indicating

its potential to combat bone demineralization, and readily

bind with bovine serum albumin. It is noted that SrCaPAEM

could release ions into the SBF and adsorbs proteins and the

release rate was able to be controlled by varying the Sr 2 +/Ca 2 +

ratio in SrCaPAEM. This ability to adsorb proteins is crucial for

future efforts in drug release control and promotion of mineral

formation.

Another heterometallic MOF, CaSr-BTC (BTC = 1,3,5-

tricarboxylicbenzene), reported by Acharya, Little and co-

workers,[106] was found to induce higher bone mineralization

(hydroxyapatite) from MC3T3 pre-osteoblastic cells than com-

pounds with individual metals (Ca-BTC and Sr-BTC). This sug-

gests that the CaSr-MOFs may release Ca 2 + and Sr 2 + from the

matrix at an appropriate rate and exhibit a cumulative effect on

osteoinduction. The subsequent assessment revealed that CaSr-

BTC were able to induce upregulation of the differentiation

markers ALP and BSP II from initial levels and cause signifi-

cant downregulation in COL1, confirming high ability to mod-

ulate osteoblast-specific mRNA levels of hMSCs. Considering

that DMOG may lead to increased vascularization, via vascular

endothelial growth factor (VEGF) production from hMSCs, the

authors prepared CaSr-DMOG-MOF by introducing DMOG to

the framework during the synthesis of CaSr-MOFs. Unexpect-

edly, though coupled with DMOG, CaSr-DMOG-MOF played a

poorer performance than that of CaSr-BTC in inducing prolif-

eration and differentiation of MC3T3 cells and hMSCs.

Nitrogen-containing bisphosphonates (N-BPs) are known to

inhibit osteoclast-mediated bone resorption and are used clini-

cally to treat patients with osteoporosis and bone metastases of

several cancers. [107] However, they are not effective anticancer

drugs due to their unfavorable pharmacokinetics, as a majority

of the injected N-BP either bind to the bones or are quickly

cleared via renal filtration. In this context, Lin and co-workers

developed a general strategy to deliver two N-BPs pamidronate

(Pam) and zoledronate (Zol) to cancer cells by incorporating

them into crystalline Ca-based nano MOF at exceptionally high

drug loadings.[108] To lower the release rate, the authors coated

the MOF particles with single lipid bilayers, and anisamide

was subsequently introduced to target receptors that are over-

expressed by many human cancer cells. [109,110] The coated MOF

particles display superior antitumor efficacy compared to the

free bisphosphonates in vitro against human lung and pancre-

atic cancer cells.

Wang and co-workers [111] reported a similar Ca-based nano-

MOF, calcium zoledronate (CaZol). They coated the nano-MOF

by polyethylene glycol (PEG) and incorporated folate (Fol)-

targeted ligands on the nano-MOF. Further in vitro and in

vivo studies demonstrated Fol-targeted CaZol nano-MOF is an

effective anticancer agent and is able to increase the direct anti-

tumor activity of Zol by 80–85% in vivo through inhibiting of

tumor neovasculature and cell proliferation and inducing apop-

tosis. In another study, Hu and co-workers[112] took advantage

of CaZol as nanocarrier for pDNA delivery. The pH-sensitive

CaZol exhibited high stability in the physiological environment

(pH 7) and the encapsulated pDNA could be released in a

weakly acidic environment (pH 5), guaranteeing a better cel-

lular uptake efficiency and desired gene expression efficiency in

vitro and in vivo.

Another pH-responsive nano-MOF, Ca/Pt(IV)@pHisPEG,

was synthesized and used as nanomedicine drug by Chen and

co-workers. [113] The nanoparticles showed prolonged blood cir-

culation and efficient passive accumulation in the tumor under

pH 7, whereas slightly acidic condition (e., pH 6) would

lead to the protonation of imidazole groups,[114,115] causing a

size expansion of the particles. Thus, tumor retention and

cellular internalization of those nanoparticles are enhanced.

Further reduced pH inside endo/lysosomes would trigger the

decomposition of those NCPs and the subsequent drug release

for effective cancer cell killing. The result of animal tumor

model demonstrated great efficacy under low drug doses, and

was found to be particularly effective toward solid tumors with

reduced pH.

Gallic acid (H4GAL) is an abundant naturally occurring

hydroxycarboxylic acid with interesting antioxidant properties,

associated with various beneficial therapeutic effects, including

anti-allergic, anti-inflammatory, antiviral, [116] antifungal, antimi-

crobial,[117] and even anticarcinogenic,[118] as well as cardio[119]

and neuro-protective[120] activities. Using H4GAL as an organic

ligand, Horcajada and co-workers [121] developed a novel 3D

Ca-MOF with fine biocompatibility, denoted as MIL-155. In

antioxidant activity experiment, MIL-155 showed a protective

activity at a low concentration (from 5 mg mL− 1 ), while its Mg-

analogue, Mg(H 2 GAL) showed antioxidant activity only at the

highest tested doses (60 mg mL− 1 ). The authors concluded that

the antioxidant effect would depend not only on concentration

of the material but also on its degradation rate. For MIL-155,

the remarkable effect is associated with the release of the gal-

late ligand into the media at an optimal rate due to the relative

weak ligand–cation bonds for MIL-155.[121] Hence, antioxidant-

based MOFs appear to be promising candidates not only for

bio-applications but also for food preservation, smart surfaces,

or cancer therapy.

Olsalazine (H 2 OLZ), a prodrug of the anti-inflammatory

5-aminosalicylic acid (5-ASA), is prescribed as the first line

of treatment for patients with idiopathic inflammatory bowel

diseases such as Crohn’s disease and ulcerative colitis. [122]

To solve the problem regarding sustained release of olsala-

zine in human body, Long and co-workers [123] synthesized

three Ca-MOFs including 1D Ca(H 2OLZ)·4H2O chains

( 1 ), 2D Ca(H 2 OLZ)·2H 2 O sheets ( 2 ), and a 3D structure

Ca(H 2 OLZ)·2DMF ( 3 ). After being pressed into pellets and

exposed to simulated gastrointestinal fluids, all three materials

exhibited a delayed release of olsalazine relative to Na 2 (H2OLZ)

which released more than 90% olsalazine. Notably, compound

3 released less than 25% of the drug, significantly lower than

that of 1 and 2 with about 50% olsalazine left. The obvious dif-

ferences of release rate for the three compounds could attribute

to their extended structures. Since compound 3 could largely

decrease the side effects associated with soluble olsalazine

in the small intestine, the authors concluded that it would

be a promising alternative to the commercial Na 2 (H2OLZ)

formulation.

Although precise supplying of drugs through controllable

degradation of bioactive MOFs is an attractive option, most of

the drugs are not able to coordinate with Ca 2 + or other cations

to form extended networks. In this context, loading drugs onto

nontoxic MOFs directly is recognized as an efficient alternative

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Review

Calcium-Based Metal–Organic Frameworks and Their

Potential Applications

Shikai Xian, Yuhan Lin, Hao Wang,* and Jing Li*

Dr. S. Xian, Y. Lin, Prof. H. Wang, Prof. J. Li

Hoffmann Institute of Advanced Materials

Shenzhen Polytechnic

7098 Liuxian Boulevard, Shenzhen, Guangdong 518055, P. R. China

E-mail: wanghao@szpt.edu.cn; jingli@rutgers.edu

Dr. S. Xian, Prof. J. Li

Department of Chemistry and Chemical Biology

Rutgers University

123 Bevier Road, Piscataway, NJ 08854, USA

The ORCID identification number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/smll.202005165.

DOI: 10.1002/smll.202005165

1. Introduction

Metal–organic frameworks (MOFs) are crystalline solids featuring

extended networks formed by coordinate bonds between inor-

ganic nodes (metal ions or clusters) and organic linkers.[1] MOFs

are characterized by their structural diversity and high tunability

with respect to porosity and surface functionality. As a relatively

new class of multifunctional crystalline materials, MOFs exhibit

considerable promise for applications across a broad range of

technologies, including gas storage,[2–5] molecular separation,[6–14]

chemical sensing,[5,15,16] heterogeneous catalysis,[17–20] and energy

efficient lighting technologies,[21–23] to name a few. For certain

applications, MOFs have outperformed some of those traditional

inorganic or organic materials, and thus, hold great promise as

their replacements or supplements. For example, over the past

few years, several MOFs have been reported to be capable of

full separation of light hydrocarbons (e.g., ethane/ethylene,

Metal–organic frameworks (MOFs) built on calcium metal (Ca-MOFs) rep-

resent a unique subclass of MOFs featuring high stability, low toxicity, and

relatively low density. Ca-MOFs show considerable potential for molecular

separations, electronic, magnetic, and biomedical applications, although they

are not investigated as extensively as transition metal-based MOFs. Com-

pared to MOFs made of other groups of metals, Ca-MOFs may be particularly

advantageous for certain applications such as adsorption and storage of

light molecules because of their gravimetric benefit, and drug delivery due to

their high biocompatibility. This review intends to provide an overview on the

recent development of Ca-MOFs, including their synthesis, crystal structures,

important properties, and related applications. Various synthetic methods

and techniques, types of building blocks, structure and porosity features,

selected physical properties, and potential uses will be discussed and sum-

marized. Representative examples will be illustrated for each type of impor-

tant applications with a focus on their structure–property relations.

ethylene/acetylene, propane/propylene)

through highly selective size exclusion

mechanism,[6,11,24] which has not been

achieved by conventional adsorbent mate-

rials. The potential uses of MOFs depend

on their pore structure (pore size/pore

shape), surface functionality, as well as the

type of metal centers and ligands.

Calcium-based metal–organic frame-

works (Ca-MOFs) represent a subgroup of

MOFs with calcium as metal centers. Unlike

MOFs built on transition and post-transition

metals (Zr, Fe, Co, Ni, Cu, Zn, etc.) which

tend to form commonly observed SBUs and

topology and, therefore allow for successful

design and implementation of targeted

structure and functionality, the prediction

of coordination geometry and structural

topology of Ca-MOFs are much more chal-

lenging. This could be attributed to the fact

that the bonding interactions between cal-

cium and organic ligands (commonly carboxylates or phosphates)

are more ionic, and thus the coordination mode of calcium and

the topology of Ca-MOFs largely rely on the nature of the organic

ligands as well as the synthetic conditions. However, Ca-MOFs

possess several advantages compared to those built on transi-

tion metals: 1) Ca-MOFs generally feature high thermal stability

because of its high electropositivity which leads to strong, ionic-

like bonds with organic ligands (e.g., carboxylates). 2) Calcium

is earth-abundant (3.4% of Earth’s crust) and nontoxic, making

Ca-MOFs relatively inexpensive and environmentally safe, thus

especially promising for biological related applications. 3) The

lightweight of calcium metal offers gravimetric benefit for gas

adsorption/storage related applications.

To date, more than 150 Ca-MOFs have been reported (Table1).

These MOFs bear different structural features and have been

evaluated for various applications, including molecular sepa-

ration, drug delivery and controlled release, chemical sensing,

and proton conductivity, to name a few. In this review, we

will present an overview of recent development of Ca-MOFs,

including their design, synthesis, crystal and pore structure,

important properties, and potential applications.

2. Synthesis and Structures of Ca-MOFs

2.1. Synthetic Methods

Similar to MOFs made of other metals, solvothermal synthesis

represents the most commonly employed preparation method

Small 2021, 17, 2005165

Calcium‐Based Metal Organic Frameworks and Their Potential Applications reff 5a

ORGANIC CHEMISTRY-I

Amity University

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Review

Calcium-Based Metal–Organic Frameworks and Their

Potential Applications

Shikai Xian, Yuhan Lin, Hao Wang,* and Jing Li*

Dr. S. Xian, Y. Lin, Prof. H. Wang, Prof. J. Li

Hoffmann Institute of Advanced Materials

Shenzhen Polytechnic

7098 Liuxian Boulevard, Shenzhen, Guangdong 518055, P. R. China

E-mail: wanghao@szpt.edu; jingli@rutgers

Dr. S. Xian, Prof. J. Li

Department of Chemistry and Chemical Biology

Rutgers University

123 Bevier Road, Piscataway, NJ 08854, USA

The ORCID identification number(s) for the author(s) of this article

can be found under doi/10.1002/smll.202005165.

DOI: 10/smll.

1. Introduction

Metal–organic frameworks (MOFs) are crystalline solids featuring

extended networks formed by coordinate bonds between inor-

ganic nodes (metal ions or clusters) and organic linkers.[1] MOFs

are characterized by their structural diversity and high tunability

with respect to porosity and surface functionality. As a relatively

new class of multifunctional crystalline materials, MOFs exhibit

considerable promise for applications across a broad range of

technologies, including gas storage,[2–5] molecular separation,[6–14]

chemical sensing,[5,15,16] heterogeneous catalysis,[17–20] and energy

efficient lighting technologies,[21–23] to name a few. For certain

applications, MOFs have outperformed some of those traditional

inorganic or organic materials, and thus, hold great promise as

their replacements or supplements. For example, over the past

few years, several MOFs have been reported to be capable of

full separation of light hydrocarbons (e., ethane/ethylene,

Metal–organic frameworks (MOFs) built on calcium metal (Ca-MOFs) rep-

resent a unique subclass of MOFs featuring high stability, low toxicity, and

relatively low density. Ca-MOFs show considerable potential for molecular

separations, electronic, magnetic, and biomedical applications, although they

are not investigated as extensively as transition metal-based MOFs. Com-

pared to MOFs made of other groups of metals, Ca-MOFs may be particularly

advantageous for certain applications such as adsorption and storage of

light molecules because of their gravimetric benefit, and drug delivery due to

their high biocompatibility. This review intends to provide an overview on the

recent development of Ca-MOFs, including their synthesis, crystal structures,

important properties, and related applications. Various synthetic methods

and techniques, types of building blocks, structure and porosity features,

selected physical properties, and potential uses will be discussed and sum-

marized. Representative examples will be illustrated for each type of impor-

tant applications with a focus on their structure–property relations.

ethylene/acetylene, propane/propylene)

through highly selective size exclusion

mechanism,[6,11,24] which has not been

achieved by conventional adsorbent mate-

rials. The potential uses of MOFs depend

on their pore structure (pore size/pore

shape), surface functionality, as well as the

type of metal centers and ligands.

Calcium-based metal–organic frame-

works (Ca-MOFs) represent a subgroup of

MOFs with calcium as metal centers. Unlike

MOFs built on transition and post-transition

metals (Zr, Fe, Co, Ni, Cu, Zn, etc.) which

tend to form commonly observed SBUs and

topology and, therefore allow for successful

design and implementation of targeted

structure and functionality, the prediction

of coordination geometry and structural

topology of Ca-MOFs are much more chal-

lenging. This could be attributed to the fact

that the bonding interactions between cal-

cium and organic ligands (commonly carboxylates or phosphates)

are more ionic, and thus the coordination mode of calcium and

the topology of Ca-MOFs largely rely on the nature of the organic

ligands as well as the synthetic conditions. However, Ca-MOFs